There are known apparatuses with these characteristics for the checking of pins rotating with orbital motion in the course of the machining in a grinding machine. For example, international patent application published with No. WO-A-9712724, filed by the same applicant of the present patent application, discloses an apparatus for the checking of the diameter of crankpins in orbital motion in the course of the machining of crankshafts in a grinding machine including a bed, a worktable, a grinding-wheel slide and a grinding wheel coupled to the grinding-wheel slide. The apparatus is coupled to the grinding-wheel slide, contacts the piece and follows it in the course of its orbital motion substantially by virtue of the force of gravity applied to the considerable mass of the apparatus. The apparatus is particularly suitable for checking crankshafts for automobile engines and has appropriate mass and layout dimensions.
Owing to the considerable layout dimensions, apparatuses of this type cannot be coupled to the grinding-wheel slide of small-size grinding machines, as those utilized for the machining of shafts for compressors, like the one (8) shown in FIG. 1, more particularly its associated eccentric pin 8′. The dimensions of these shafts are by far smaller than those of the crankshafts: a shaft for compressors is typically 150-200 mm long and the eccentric pin is approximately 12-40 mm in diameter, while a crankshaft measures at least 50-100 cm in length and the diameter of a crankpin may range, for example, within 40 to 90 mm. In order to carry out the diameter checking, during the machining of these eccentric pins, the presently utilized applications are substantially similar to the one illustrated and described in italian patent No. 1258154. These applications (an example is shown in simplified form in FIG. 2) include two stationary gauging or measuring heads H1 and H2, coupled to the machine bed B or to the worktable, with feelers for contacting the pin, in the course of its eccentric rotation, just at two diametrally opposite points, P1 and P2, of its trajectory T. The diameter of the pin is calculated by evaluating information relating to the position of said two points of the trajectory and carrying out appropriate processings that keep into account the geometry of the checked piece.
Even though the utilization of a checking application of this type is simple, it cannot guarantee satisfactory metrological performances because, among other things, the diameter of the pin is “deduced” on the basis of checkings carried out by touching the same point of the surface in two opposite arrangements of the piece.
It is not possible to determine whether any possible variations detected by either of the two heads is due to diameter variations, to shape and/or eccentricity errors or to a combination of such factors. Furthermore, the measurement combining the detections of the two heads is also affected by the mutual arrangement existing between the heads, and by possible modifications of said arrangement. Furthermore, the detecting and processing operation is slow and, whenever the nominal diameter dimensions of the piece to be checked vary, it is necessary to manually reset the application and consequently this implies machine down-time and considerable loss of time.